In many processes, for instance, in the field of industrial processing engineering, chemistry or machine construction, a gas mass, especially an air mass, has to be supplied in a specific manner. Among these are especially combustion processes, which run under regulated conditions. In this context, an important example is the combustion of fuel in internal combustion engines of motor vehicles, especially having subsequent catalytic exhaust gas purification. Various types of sensors are used, in this instance, to measure the air-mass throughput.
One sensor type from the related art is the so-called hot-film air-mass sensor (HFM), which is discussed in one specific embodiment, for instance, in DE 196 01 791 A1. Usually, in such hot-film air-mass sensors, a thin sensor diaphragm is applied on a sensor chip, for instance, a silicon sensor chip. On the sensor diaphragm there is typically situated at least one heating resistor, which is surrounded by two or more temperature measuring resistors. In an air flow, that is conducted over the diaphragm, the temperature distribution changes which, in turn, may be recorded by the temperature measuring resistors. Consequently, an air mass flow may be determined, for example, from the difference in resistance of the temperature measuring resistors. Various other variations of this sensor type are known from the related art.
One difficulty with this type of sensor, for example, from DE 101 11 840 C2 is that contamination of the sensor chip, such as with oil, may frequently occur. The sensor chip is normally used either directly in the intake tract of the internal combustion engine, or in a bypass of the intake tract of the internal combustion engine. Even in the operation of the internal combustion engine, oil may deposit on the sensor chip and especially on the sensor diaphragm, in this instance. This oil deposit may lead to an undesired effect on the measuring signal of the sensor chip, especially since an oil film on the surface of the sensor chip has an effect on the thermal conductivity of the surface, which leads to a corruption of the measuring signals.
This oil contamination appears particularly during, or shortly after the shutting down of the internal combustion engine, for instance, of a Diesel engine. After shutting down the internal combustion engine, an overpressure present in a crankcase typically becomes reduced via a crankcase ventilation into the intake tract of the internal combustion engine. In this context, oil vapor or oil mist is frequently carried along, which is in a position to deposit as an oil film in the intake tract, and thus also on the air-mass flow sensor situated there (or in a corresponding bypass).
German patent document no. DE 101 11 840 C2 therefore proposes a sensor chip which has a frame element, for instance, of silicon, having a diaphragm applied onto it. Various metal traces are situated on the diaphragm, which function as electric heaters and/or measuring resistors, whereby the region of the diaphragm forms a sensor region. Moreover, on the surface of the sensor chip, in addition, at least one additional heater is situated, which may be heated electrically in such a way that, in the region of the additional heater, thermogradient turbulences are formed, which lead to the depositing of contamination in the region of the additional heater, out of the way of the region of the sensor region.
However, this device from the related art has various disadvantages. A substantial disadvantage is that, in DE 101 11 840 C2, the additional heater is situated directly near or even on the sensor diaphragm. In the device described, this is conditioned particularly by the silicon sensor chip having only a small surface, so that a positioning of additional heater and sensor diaphragm at a distance has to be implemented technically. In the case of heavy oil loading, this adjacent positioning of additional heater and sensor diaphragm can, however, lead to oil droplets flowing back onto the sensor diaphragm, and can consequently, in turn, lead to contamination of the sensor diaphragm, and to a change in thermal conductivity connected therewith, and consequently to a signal drift.
Furthermore, one disadvantage of the situation shown in DE 101 11 840 C2 is the so-called Marangoni effect, which is the development of different surface tensions at different temperatures. An oil film has a greater surface tension with respect to air in a cold state than in a hot state. If there is a thermal gradient in a liquid, the Marangoni effect usually leads to a fluid motion from warmer zones to colder zones. Consequently, the device in DE 101 11 840 C2 may even have the disadvantage that oil droplets on the surface of the sensor chip are pushed from the additional heater to the sensor diaphragm, and thus the contamination of the sensor diaphragm is even increased, instead of lowered.
An additional disadvantage of the device from the related art is that the main contamination by oil, as explained above, appears at, or shortly after the shutting down of the internal combustion engine, since, at that time, oil mists diffuse through the intake tract and the bypass all the way to the hot-film air-mass sensor, and contaminate it. The devices known from the related art thus have the disadvantage that, immediately after shutting down the internal combustion engine, the hot-film air-mass sensor is strongly contaminated with oil.